Local first law of gravity

We find a (quasi)local first law of thermodynamics, ΔE=TΔS-W, connecting gravitational entropy, S, with matter energy and work. For Einstein gravity, S is the Bekenstein-Hawking entropy, while for general theories of gravity, S is the Wald entropy, evaluated on the stretched future light cone of any point in an arbitrary spacetime, not necessarily containing a black hole. The equation can be written as ρΔV=TΔS-pΔV by regarding the energy-momentum tensor as that of a fluid.